The invention refers to a method for evaluating a capacity value of a capacitive sensor electrode.
The use of capacitive sensor electrodes is known in practice for various applications.
An example of the use of capacitive sensor electrodes are capacitive proximity switches, which are mounted on vehicles, in order to detect the presence of a person, for example. The function of such a capacitive proximity sensor is based on the fact that the capacity of a capacitive sensor electrode of the proximity sensor reaches a certain value or exceeds the same or experiences a predetermined absolute or relative change.
In order to evaluate the capacity at a certain time or during a specific time interval, the capacitive sensor electrode is coupled to a predetermined potential. The sensor electrode forms a capacity with a reference electrode. The reference electrode, in the example of a proximity sensor on a vehicle, may be a vehicle ground or a ground of the terrain under the vehicle. If a body part of a user, such as a hand or a foot, enters the sensitive area of the sensor electrode, the electrostatic properties of the entire system are changed. In the context of an equivalent observation, this effect may be considered as a change of the dielectric, which surrounds the sensor electrode.
In order to derive, from the capacity value of the sensor electrode or from the change of the capacity value of the sensor electrode, a piece of information such as a piece of information regarding the presence of a hand or a foot, an evaluation of the capacity value is required.
The evaluation of the capacity value may occur in different ways. For example, a repeated charging of the sensor electrode to a reference potential of a voltage source and discharging of the sensor electrode into a comparative capacity and after the repeated charging and discharging, an evaluation of the charge accumulated on the comparative capacity may be performed. An example of such an implementation of this concept is shown in DE 196 81 725 B4.
Fundamentally, in order to evaluate the capacity value, the principle is used, according to which the sensor electrode is periodically charged and, after the periodic charging, a capacity dependent parameter, either of the charging process or of the discharging process, which is generally also periodically performed, is evaluated.
The parameter to be evaluated may be a voltage, for example, which is measured over a capacitor collecting a charge, such as the one described in the previous document.
Another possible parameter to be evaluated may be a number of charging and discharging cycles until a switch threshold on a compensation capacitor is exceeded. An example of such a procedure is provided in DE 10 2013 112 910 A1. In the method described in this publication, during a first phase, the capacitive sensor electrode is coupled via a first switching device to a charging voltage, in order to charge the sensor electrode. At the same time, a compensation capacity is introduced by means of a further second switching device between a reference voltage and the ground, whereby the compensation capacity is also provided with a charge. Thereafter a charge compensation between sensor electrode and compensation capacity is performed. The determination of the electrode capacity occurs based on an evaluation of the charge accumulated on the compensation capacity.
It is thus to be noted that methods known in the art and from practical use, for evaluating the capacity value of a capacitive sensor electrode, require that the sensor electrode is cyclically charged. Depending on the concrete embodiment, the sensor electrode may be charged stepwise in a plurality of cycles, or the sensor electrode in each cycle of a plurality of cycles, may be both charged—partially or completely—and discharged—partially or completely.
In order to ensure a reliable evaluation of the capacity of the sensor electrode in a sufficiently short time, the cyclical charging and possibly also the discharging of the sensor electrode occur generally at relatively high frequencies. Frequencies are often in the range from some 10 kHz to several 100 kHz.
Due to the high-frequency charging of the sensor electrode, the problem arises—tendentially more pronounced at higher frequencies—that due to the high-frequency cyclical charging and possible discharging, undesired alternate electromagnetic fields are generated. Depending on the size and shape of the sensor electrode, the circuital implementation of the control of the sensor electrode and of the selected frequency, the sensor electrode correspondingly behaves as an antenna and irradiates electromagnetic radiation. This effect may be particularly problematic due to the fact that products irradiating electromagnetic waves are subject to normative limitations. For example, in the Federal Republic of Germany, limitations based on the federal transmission protection law are in effect, according to which compulsory limits for electromagnetic radiations may be derived. The corresponding limiting values are frequency-dependent and relatively tight. The frequency used for evaluating sensor electrodes, as said, which are typically relatively high, on one hand, and the strict normative requirements, on the other hand, set high requirements for the construction and the operation of capacitive sensor electrodes.
Based on these background observations, the object of the invention is to allow the evaluation of a capacitive value of a capacitive sensor electrode and the arrangement of a capacitive sensor electrode, such as on a motor vehicle, with improved flexibility, for example with regards to configuration and mounting site.